Compared to other widely used drugs (alcohol, tobacco, opiates)
a smaller percentage of cannabis users become dependent. Dependency
is also less severe compared to many other legal and illegal
drugs. The relatively low dependence liability of cannabis
is widely recognized.

Withdrawal from THC has been described in animal research
and humans. For example, people who smoke marijuana daily
become more aggressive when they quit. Dr. Elena Kouri and
colleagues at Harvard University write in the Journal of Psychopharmacology
that they had shown objectively that when people stop smoking
marijuana, there is a clear withdrawal syndrome (Kouri and
Pope 2000).

The withdrawal symptoms are relatively mild. In a review
of the published literature regarding cannabis withdrawal
symptoms in humans, Smith (2002) stated:

"It is suggested that the studies conducted to date
do not provide a strong evidence base for the drawing of any
conclusions as to the existence of a cannabis withdrawal syndrome
in human users, or as to the cause of symptoms reported by
those abstaining from the drug. On the basis of current research,
cannabis cannot be said to provide as clear a withdrawal pattern
as other drugs of abuse, such as opiates. However, cannabis
also highlights the need for a further defining of withdrawal,
in particular the position that rebound effects occupy in
this phenomenon. It is concluded that more controlled research
might uncover a diagnosable withdrawal syndrome in human users
and that there may be a precedent for the introduction of
a cannabis withdrawal syndrome before the exact root of it
is known" (Smith 2002).

Tolerance and rebound phenomena in humans have been described
for cannabis. These are other indications of dependency caused
by cannabis:

"Tolerance develops to the receptor-mediated effects
of THC with continued usage. However, there are distinctions
in their degree with different effects. Discontinuation of
chronic THC use may cause rebound phenomena (transient increase
in intraocular pressure, loss of appetite, etc.). Some chronic
users report withdrawal symptoms after abrupt cessation. This
withdrawal syndrome is characterized by irritability, agitation,
sleep disorder, hyperhidrosis and loss of appetite. It is
generally mild. Cannabis dependency is less determined by
physical than by psychological factors. Dependency and abuse
potential of therapeutically employed ?9-THC is low"
(Grotenhermen 2002).

Dependency rates are reported to be lower than with many
other drugs. In a German study of 1,458 current or previous
cannabis users, ordered by the German Federal Health Ministry,
2-10% of those using exclusively cannabis were classified
as substance dependent (Kleiber et al. 1997). If those who
also used other illegal drugs were included, 8% of cannabis
users were regarded as dependent, including 1% of the "occasional
users," 7% of the "individual users," 10% of
the "recreational users," and 28% of the "permanent
users." Duration of consumption had no influence on the
likelihood of the subject to quit use, an indication that
the risk of dependency was independent of duration of use,
and that users generally had no problems quitting.

Similar percentages were reported by Hall et al. (1999):

"A variety of estimates have been derived from U.S.
studies in the late 1970s and early 1980s, which defined cannabis
use and dependence in a variety of ways. These studies suggested
that between 10 and 20 per cent of those who have ever used
cannabis, and between 33 and 50 per cent of those who have
had a history of daily cannabis use, showed symptoms of cannabis
dependence (see Hall, Solowij & Lemon, 1994). A more recent
and better estimate of the risk of meeting DSM-R.III criteria
for cannabis dependence was obtained from data collected in
the National Comorbitity Study (Anthony, Warner & Kessler,
1994). This indicated that 9 per cent of lifetime cannabis
users met DSM-R-III criteria for dependence at some time in
their life, compared to 32 per cent of tobacco users, 23 per
cent of opiate users and 15 per cent of alcohol users"
(Hall et al. 1999)

In the recent past, several studies have attempted to compare
the health risks of the most common legal and illegal drugs.
Two studies received special attention: a report by order
of the French Health Ministry, the so-called "Roques-Report"
(Roques 1998), and a study prepared for the World Health Organization
(Hall et al. 1999). Major attention was paid to dependency/addiction
caused by these drugs. The main results of these studies are
summarized in Tables 1 and 2 below.

Table 1. Comparison of hazards of different drugs (modified
according to Roques et al. 1998).

Table 2. Comparison of adverse effects on health for heavy
users of the most harmful common form of each substance(according
to Hall et al. 1999).

Marijuana

Alcohol

Tobacco

Heroin

Traffic and other accidents

*

**

*

Violence and suicide

**

Overdose death

*

**

HIV and liver infections

*

**

Liver cirrhosis

**

Heart disease

*

**

Respiratory diseases

*

**

Cancer

*

*

**

Mental illness

*

**

Dependency/addiction

**

**

**

**

Lasting effect on the
fetus

*

**

*

*

* = less common or less well-established effect

** = important effect

Both reports concluded that heavy cannabis consumption
causes less harm than heavy use of the most common other
legal and illegal drugs. Special attention was paid to the
question of dependency and abuse. Hall et al. (1999) concluded
that all drugs under investigation can cause dependency.
The main health risks to exclusive users of cannabis would
be limited to daily users who consume the drug over a period
of several years. These risks included the risk of a dependency
syndrome, development of a chronic bronchitis, and involvement
in motor vehicle accidents if the user drives under acute
influence of the drug. The latter could also affect occasional
users. With regard to dependency Hall et al. (1999) conclude
(as quoted above):

"A variety of estimates have been derived from U.S.
studies in the late 1970s and early 1980s, which defined
cannabis use and dependence in a variety of ways. These
studies suggested that between 10 and 20 per cent of those
who have ever used cannabis, and between 33 and 50 per cent
of those who have had a history of daily cannabis use, showed
symptoms of cannabis dependence (see Hall, Solowij &
Lemon, 1994). A more recent and better estimate of the risk
of meeting DSM-R.III criteria for cannabis dependence was
obtained from data collected in the National Comorbidity
Study (Anthony, Warner & Kessler, 1994). This indicated
that 9 per cent of lifetime cannabis users met DSM-R-III
criteria for dependence at some time in their life, compared
to 32 per cent of tobacco users, 23 per cent of opiate users
and 15 per cent of alcohol users" (Hall et al. 1999).

Eminent addictions specialist Jack Henningfeld was asked
to rate the addictive qualities of popular drugs for the
New York Times, and produced the following ratings according
to five general indicators of abuse potential.

Reinforcement: A measure of the substance's ability, in human
and animal tests, to get users to take it again and again,
and in preference to other substances.

Tolerance: How much of the substance is needed to satisfy
increasing cravings for it, and the level of stable need that
is eventually reached.

Dependence: How difficult it is for the user to quit, the
relapse rate, the percentage of people who eventually become
dependent, the rating users give their own need for the substance
and the degree to which the substance will be used in the
face of evidence that it causes harm.

Intoxication: Though not usually counted as a measure of
addiction in itself, the level of intoxication is associated
with addiction and increases the personal and social damage
a substance may do. (Heningfeld, Hilts, 1994)

This assessment agrees with those cited above in that marijuana
ranks low on all indicators of additive potential compared
to other commonly used drugs.

Adolescents are much more susceptible to marijuana dependence
and to problems related to marijuana abuse than adults.

“Adolescents are dependent at a lower frequency and
quantity of use than adults: the differences diverge as level
of use increases. Twice as many adolescents as adults who
used marijuana near-daily or daily within the last year were
identified as being dependent (35% versus 18%). Frequency
and quantity of use each retained a unique effect on dependence,
but frequency appeared to be more important than quantity
in predicting last year dependence.” (Chen et al, 1997)

This higher dependence liability of adolescents is sometimes
used as an argument against the medical use of cannabis. However,
this argument is not used with other medicines, such as the
opiates. The IOM report states that permitting the medical
use of marijuana would not increase non-medical uses. The
report also addresses the suggestion by opponents of medical
use that approving marijuana as a medicine "sends the
wrong message." The authors say there is "no convincing
data to support this concern," and they note that "this
question is beyond the issues normally considered for medical
uses of drugs." (Joy et al. 1999).

Kandel et al. (1997) analyzed dependency rates in a sample
of about 88,000 individuals. They found that nicotine was
the most addictive drug. Analyses were based on three aggregated
waves (1991, 1992 and 1993) of the nationally representative
samples of the general population, at or above 12 years of
age, interviewed in the National Household Surveys on Drug
Abuse (n = 87915).

"The five major findings are that: (1) nicotine is the
most addictive of the four drugs we examined; (2) among female
last year users of alcohol and marijuana, adolescents are
significantly more at risk for dependence than any other age
group of women; (3) conditional prevalences of last year dependence
on alcohol, marijuana and cocaine are higher among adolescent
females than adolescent males but significantly different
only for cocaine; (4) among adults, the rates of dependence
are higher among males than among females for alcohol and
marijuana, but lower for nicotine; and (5) among last year
users, whites are more likely than any other ethnic group
to be dependent on nicotine and blacks to be dependent on
cocaine" (Kandel et al. 1997).

If selected samples of individuals are investigated, it is
necessary to avoid any generalization of the results. Crowley
et al. (1998) investigated a sample of young cannabis users
(age: 13-19 years) with serious cannabis-use disorders and
problems and noted:

"The prevalence of cannabis use is rising among adolescents,
many of whom perceive little risk from cannabis. However,
clinicians who treat adolescent substance users hear frequent
reports of serious cannabis-use disorders and problems. (...)

The data indicate that for adolescents with conduct problems
cannabis use is not benign, and that the drug potently reinforces
cannabis-taking, producing both dependence and withdrawal.
However, findings from this severely affected clinical population
should not be generalized broadly to all other adolescents."

In conclusion, cannabis can cause dependency but withdrawal
is milder than withdrawal from several other legal and illegal
drugs, and dependency is less frequent than with most other
common legal and illegal drugs.

References

Chen K, Kandel DB, Davies M. Relationships between frequency
and quantity of marijuana use and last year proxy dependence
among adolescents and adults in the United States. Drug Alcohol
Depend 1997 Jun 6;46(1-2):53-67